Fix expanded plot layout

[happykhan]

What changed

• centre the circular map when entering expanded view while preserving zoom and legend positions
• visually distinguish the square white canvas from the surrounding expanded workspace
• add an ultrawide 2048 x 722 regression test that begins from an off-centre dragged plot

Root cause

Expanded view retained the inline plot's pan offset. The wide fullscreen wrapper also used the same white background as the square canvas, so the clipped map and unused workspace appeared to be one broken canvas.

Validation

• lint, architecture, licence and security checks
• 176 unit tests
• production build
• 23 production browser tests
• Vite development-mode BLAST/WASM browser test

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